While the present invention is applicable to a variety of rotation sensing uses and environments, one specific application of the present invention is as a wheel speed sensor system in automotive and truck vehicles. It provides an output signal suitable for driving an antilock system on such a vehicle.
Examples of prior wheel speed sensing systems may be seen in U.S. Pat. Nos. 3,854,556, 3,938,112, 3,961,215 and 4,029,180, assigned to the Assignee of the present invention. Such systems typically mount a rotor on the wheel of which the speed is to be sensed, and a speed sensing device is fixed with respect to the axle housing. The sensing device is typically of electromagnetic type, whereas the rotor is of ferromagnetic material and is provided with surface interruptions such as apertures, teeth or ripples.
The aforementioned systems have provided the rotor and sensing device with interacting radial portions axially opposed across an air gap. Variable reluctance-type sensing is achieved by providing a magnetic flux source, in the form of a magnet, in the sensing device to establish a flux path through pole pieces extending from the magnet to the radial surface of the sensing device, across the air gap, and into the slotted radial portion of the rotor. Rotation of the rotor moves the slots past the pole pieces to periodically change the magnetic flux in the path, which change is sensed by a coil wound around the magnet to provide a corresponding electrical output signal of frequency related to the rotation rate of the rotor. Since such devices are dependent solely on flux change for generation of an output signal, it is important that the flux change resulting from rotation of the rotor be significantly larger than any flux change resulting from radial or axial movement of the rotor relative to the sensing device, since these latter flux changes cause signal aberrations normally characterized as "noise". Thus, to obtain an accurate indication of the angular velocity of the rotating body, it is necessary that the sensing system have a high signal-to-noise ratio. This is particularly important when the wheel speed sensor provides the wheel speed signal for a vehicle antilock system, so that a skid condition can be detected and corrected in a proper manner.
Various of the aforementioned patents propose structures for obtaining an improved signal-to-noise ratio. A difficulty in such prior structures in presented by the requirement that the air gap be minimized in order to maximize the electrical output signal from the sensing device. However, as the air gap is reduced in the interest of greater system sensitivity, the more noticeable, in comparison, become even relatively moderate axial and/or radial movement of the rotor as it rotates relative to the sensing device, as due to cocking of the rotor with respect to its rotational axis, or flexing thereof during operation.
Accordingly, the objects and purposes of this invention include provision of:
(1) A rotation sensing system particularly for sensing the speed of a vehicle wheel, and intended to provide an increased signal-to-noise ratio.
(2) A system, as aforesaid, providing a rotor and opposed sensing device configured to reduce sensitivity to radial and axial motion of the rotor relative to the sensing device.
(3) A system, as aforesaid, in which the interacting faces of the rotor and sensing device, opposed across the air gap, are relatively large for permitting a relatively large air gap thickness without reducing system sensitivity, and for providing a more favorable ratio as between air gap thickness and expected radial and/or axial vibration or runout of the rotor.
(4) A system, as aforesaid, wherein the effect of rotor runout toward and away from the sensing device is substantially reduced by a tapered interfitting configuration of the rotor and sensing device, and wherein relative axial displacement of said rotor and sensing device results, in proportion of such taper, in a much smaller change in the thickness of the air gap.
(5) A system, as aforesaid, in which a two-legged air gap as provided between the tapered sensing device and rotor, cancels the effect of rotor runout in a direction transverse to such air gap, due to a reduction in the thickness of one air gap leg which is compensated for by a corresponding increase in the thickness of the other air gap leg, whereby the effective output signal is not disturbed.
(6) A system, as aforesaid, in which a rotor of relatively simple configuration is readily formable in relatively thin sheet metal while obtaining sufficient rigidity and resistence to deflection during rotation, by reason of its cross sectional configuration.
(7) A system, as aforesaid, which permits an improved signal-to-noise ratio despite use of only a single coil and substantially a single flux path.
Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.